The present invention concerns tuners for integrated continuous-time filters, particularly tuners for MOSFET-C filters in the reach channel of a disk drive.
Filters are input-output devices that reject or pass signals based on their frequency content, that is, the distribution of their energy across a range of frequencies. Every filter has a pass band that defines its effect on the frequency content of input signals. For example, low-pass filters pass signal frequencies below a certain frequency known as a cut-off frequency, and reject frequencies above the cut-off frequency, whereas high-pass filters pass frequencies above a cut-off frequency. Band-pass filters pass frequencies that are between lower and upper cut-off frequencies.
In addition to their pass-band characteristics, filters are also characterized according to their construction and temporal operation. Filters typically comprise a network of resistors and capacitors and transistors, which defines the pass band, or frequency response. When most of these components are fabricated on a common substrate, or foundation, the filter is known as an integrated filter. As for temporal operation, filters operate either in continuous time, which allows their inputs and outputs to change at any time, or in discrete time, which allows changes at only specific, time increments.
One common problem with integrated, continuous-time filters is that the effective values of capacitors and resistors varies not only because of changes in operating temperature but also because of process variations in building integrated circuits. The changes in effective value of these components causes the filter to reject or pass undesirable signals, ultimately preventing the filters from operating as intended. Compensating for fabrication-related variability and on-going component changes requires "tuning," or adjusting, the value of one or more filter components, not only initially to establish the desired frequency response, but also continually to maintain the desired frequency response of the filter throughout its operation.
One solution to the component variability problems is to use a specific type of filter known as MOSFET-C filters, which combine capacitors with metal-oxide-semiconductor, field-effect transistors (MOSFET) used as voltage-controlled resistors. In such filters, tuning, or adjusting, a control voltage of the MOSFET varies the resistance and thus tunes frequency response. In conventional tuning schemes for MOSFET-C filters, the control voltage is adjusted using a phase-locked-loop-based tuning circuit, which comprises not only a phase detector and a charge pump, but also a voltage-controlled oscillator. Although these schemes provide accurate frequency-response control, they do so only at the expense of circuit complexity and size.
Accordingly, there is a continuing need for alternative tuning schemes which are simple, yet allow accurate tuning of integrated, continuous-time MOSFET-C filters.